Efficiently removing ink from secondary fiber without impacting fiber quality is one of the major challenges in paper recycling. Currently, the most widespread method of removing ink from secondary fiber is an alkaline process that uses sodium hydroxide, sodium silicate, hydrogen peroxide, surfactants and chelants. The caustic is used to elevate the pH in the repulper causing the fiber to swell, which assists in ink detachment, but also yellows the fiber due to interaction with lignin in mechanical grades, resulting in a brightness loss. Peroxide is added to reduce fiber yellowing, and chelant is added to prevent peroxide degradation by metals. Surfactants are used to manage the detached ink and prevent redeposition onto the fiber.
While the conventional method of deinking is effective for ink removal, it has disadvantages. When the cost of the chemicals needed to overcome the unwanted effects of caustic is considered, the alkaline method is quite expensive. Aside from the high cost of the chemicals used, handling caustic can be hazardous, and it is critical to maintain the proper balance of caustic, peroxide and silicate to produce fiber with the desired optical properties. Moreover, any residual fiber yellowing, or chromophoric generation, that is caused by caustic and cannot be removed with bleaching is balanced blue dye to the fiber. While this is effective for achieving a neutral color balance, it reduces ISO brightness, making it difficult to reach brightness targets. In addition, a recent study showed that fiber strength and tensile index was reduced for secondary fiber deinked under alkaline conditions. Finally, the elevated pH in the pulper saponifies adhesives, thereby introducing more stickies into the papermaking system, which cause runnability problems for the paper machine.
An improved methodology is therefore desired.